In the manufacture of printed circuit boards for industrial use, often literally thousands of small pin holes must be drilled into each circuit board so that appropriate electronic components may be mounted into the pin holes and soldered onto the board. In volume production of the circuit boards, the drilling of holes is accomplished by computer-controlled automatic drilling machines in which the printed circuit boards are usually situated on a worktable movable in a horizontal X-Y plane. The worktable is moved horizontally in an incremental fashion beneath a drill spindle so that the pin holes may be drilled at proper locations in the circuit boards by movement of the drill spindle through a vertical drilling stroke. This is a very rapid and satisfactory procedure for mass production of printed circuit boards. However, many printed circuit boards require the drilling of holes having several different diameters. Consequently, drill tools having tips of varying diameters must be employed during the drilling of each such board. In addition, drill tools rapidly wear out and therefore need frequent replacing. In both cases, the drilling of holes must stop until one drill tool is removed from the drill spindle and a new drill tool of the same or different size is mounted in the drill spindle.
In order to facilitate the replacement of drill tools, apparatuses have been developed for automatically changing the drill tool. Such a device is shown, for example, in U.S. Pat. No. 3,973,863 to Smith. That patent discloses a drill tool transfer device having a movable transfer collet comprising a slotted sleeve which is automatically positioned below a drill spindle holding a drill tool which is to be replaced. The transfer collet is provided with a means for gripping a plastic collar on the shank of the drill tool. The collet grips the collar and then returns the drill tool to a drill tool magazine mounted on the drilling machine. A new drill tool is then taken by the sleeve of the tool transfer device and mounted in the drill spindle, thereby allowing drilling to continue. The advent of automatic drill replacement has resulted in a substantial reduction in the time it takes to replace a spindle drill tool, and thus has significantly decreased drilling time.
An additional advantageous development in circuit board drilling technology is the use of a pressure foot normally formed on the bottom of the drill spindle assembly. In use, the pressure foot tightly holds down a stack of circuit boards in a fixed position during drilling of the boards. The pressure foot prevents movement of the boards and prevents the formation of burrs in the boards which arise around the drill site during drilling, and also improves the accuracy of the holes drilled in the circuit boards. The pressure foot desirably includes a ring-shaped contact element which tightly presses down on the circuit boards during the drill stroke. The drill tool passes through a center opening in the contact element during the drill stroke.
It has been found that the efficacy of the pressure foot in reducing burrs and in improving hole accuracy is greatly increased where the contact element of the pressure foot is disposed in close proximity around the drill tool. In most cases, however, the preferable diameter of the center opening in the contact element is too small to permit the drill tool transfer device to pass through it and into the pressure foot to effect a drill tool change. As a result, in order to accommodate the need for automatically replacing drill tools while at the same time maintaining small contact elements, removable pressure foot inserts are used which mount into the bottom of the pressure foot. The pressure foot insert is removed prior to a drill tool change to allow the transfer collet on the drill tool transfer device to access the drill tool on the spindle. The insert is then remounted in the drill spindle after the tool change has taken place.
Additionally, because it is advantageous that the diameter of the center opening of the contact element be only slightly larger than the diameter of the drill tool so that the contact element applies pressure near to the drill site, it is desirable to provide a number of pressure foot inserts having center openings of various sizes to accommodate different drill tool sizes. The size of the insert opening depends upon the size of the drill tool used. For example, if a large diameter drill tool is used, a pressure foot insert having a correspondingly large diameter center opening is mounted in the bottom of the pressure foot to allow the drill tool to pass through the center opening. Similarly, if a small drill tool is used, an insert having a correspondingly small diameter center opening is used in the pressure foot.
The pressure foot insert must be removed and replaced any time that a drill tool change occurs, whether because a drill tool having a different diameter is needed or because the currently used drill tool is worn out and requires replacing. Manual removal and reinsertion of the pressure foot insert requires stopping the drill machine and is therefore unacceptably slow. Therefore, there is a need for a device and method for automatically and rapidly removing a pressure foot insert from a pressure foot during the drilling of printed circuit boards and for replacing the insert after a drill tool change. Further, there is a need for a device which can change the pressure foot insert in response to a change in the diameter of the drill tool.